DEPTH OF WELLS. 185 



up, but considerably increased the production, the necessity of 

 deepening the older wells was less pressing, and we see, that since 

 1893 their number has only been increased by 17. 



The steadiness of the number of middle class wells, viz., those 

 between 200 and 250 feet, on which as we shall presently see, the 

 production chiefly depends, is remarkable. The only increase is 

 recorded between 1888 and 1891, when 44 were added to their 

 number. These wells, which derive their supply from the 3rd oil sand 

 remain therefore for a longer time productive without the necessity 

 of being increased in depth, and whatever number is taken off by the 

 wells becoming deeper is added by the same number of shallow wells 

 advancing to the middle class stage. 



In discussing the shallow wells I stated that they showed a 

 small decrease of 20 in 1895 as compared with 1893; now if we 

 add this number to the total of the III and IV class wells, we 

 obtain the number of 304 or nearly exactly the same figure to 

 which the sum of the III, IV and V class wells amounts in 1895, viz., 

 306, or, in other words, the depth of the wells has been only slightly 

 increased since 1893, no necessity having arisen for doing so. 



2. Production of the pit wells.— In my two previous papers I 

 classified the productive wells into six classes according to the 

 quantity of petroleum each well yielded per day. This same clas- 

 sification will be again followed as it has been proved the most 

 convenient form to examine the pit-wells from this point of view. 

 The wells are, therefore, divided into— - 



1st class wells yielding less than 21 viss per day. 

 2nd „ „ „ from 21 to 50 „ „ „ 



3rd „ „ „ „ 51 to 100 „ „ „ 



4 th 1 01 to 150 



5 th 151 to 300 



6'h .... more than 300 



( 2 3I ) 



